Poor bad service from Grand Central Motors

We bought a Mercedes Benz C180, the service went well until our last service. The kilos were on 90000 and we took the car in for the brakes and we asked them to do the last service which was due. Mercedes Benz told us that they will do the brakes only. They cannot do the service if the vehicle hasn't shown us that it's due for service. So they refused to service the car. Fast forward, the sign came up to service the car however the kilos were over 100,000. And they have refused to service it while we still have 1 service remaining before the service plan finishes. We have called head office and tried to get this resolved as they now want us to pay over R13000 for the service. We have not even received feedback and now the kilos have also went up while we were waiting. This is bad service and we would like to know why Mercedes Benz returned us to only later refuse to service the vehicle.